Internet Engineering Task Force (IETF) W. Kumari
Request for Comments: 6472 Google, Inc.
BCP: 172 K. Sriram
Category: Best Current Practice U.S. NIST
ISSN: 2070-1721 December 2011
Recommendation for Not Using AS_SET and AS_CONFED_SET in BGP
Abstract
This document recommends against the use of the AS_SET and
AS_CONFED_SET types of the AS_PATH in BGPv4. This is done to
simplify the design and implementation of BGP and to make the
semantics of the originator of a route more clear. This will also
simplify the design, implementation, and deployment of ongoing work
in the Secure Inter-Domain Routing Working Group.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6472.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Kumari & Sriram Best Current Practice [Page 1]

RFC 6472 AS_SET, AS_CONFED_SET Use Deprecation December 2011
Table of Contents
1. Introduction ....................................................22. Requirements Notation ...........................................33. Recommendation to Network Operators .............................34. Security Considerations .........................................45. Acknowledgements ................................................46. References ......................................................46.1. Normative References .......................................46.2. Informative References .....................................41. Introduction
The AS_SET path segment type of the AS_PATH attribute (Sections 4.3
and 5.1.2 of [RFC4271]) is created by a router that is performing
route aggregation and contains an unordered set of Autonomous Systems
(ASes) that the update has traversed. The AS_CONFED_SET path type
([RFC5065]) of the AS_PATH attribute is created by a router that is
performing route aggregation and contains an unordered set of Member
AS Numbers in the local confederation that the update has traversed.
It is very similar to AS_SETs but is used within a confederation.
By performing aggregation, a router is, in essence, combining
multiple existing routes into a single new route. This type of
aggregation blurs the semantics of what it means to originate a
route. Said aggregation can therefore cause operational issues, such
as not being able to authenticate a route origin for the aggregate
prefix in new BGP security technologies (such as those that take
advantage of the "X.509 Extensions for IP Addresses and AS
Identifiers" [RFC3779]). This in turn would result in reachability
problems for the aggregated prefix and its components (i.e., more
specifics). Said aggregation also creates traffic engineering
issues, because the precise path information for the component
prefixes is not preserved.
From analysis of past Internet routing data, it is apparent that
aggregation that involves AS_SETs is very seldom used in practice on
the public network [Analysis] and, when it is used, it is usually
used incorrectly -- reserved AS numbers ([RFC1930]) and/or only a
single AS in the AS_SET are by far the most common case. Because the
aggregation involving AS_SETs is very rarely used, the reduction in
table size provided by said aggregation is extremely small, and any
advantage thereof is outweighed by additional complexity in BGP. As
noted above, said aggregation also poses impediments to
implementation of said new BGP security technologies.
Kumari & Sriram Best Current Practice [Page 2]

RFC 6472 AS_SET, AS_CONFED_SET Use Deprecation December 2011
In the past, AS_SET had been used in a few rare cases to allow route
aggregation where two or more providers could form the same prefix,
using the exact match of the other's prefix in some advertisement and
configuring the aggregation differently elsewhere. The key to
configuring this correctly was to form the aggregate at the border in
the outbound BGP policy and omit prefixes from the AS that the
aggregate was being advertised to. The AS_SET therefore allowed this
practice without the loss of BGP's AS_PATH loop protection. This use
of AS_SET served a purpose that fell in line with the original
intended use. Without the use of AS_SET, aggregates must always
contain only less specific prefixes (not less than or equal to), and
must never aggregate an exact match.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Recommendation to Network Operators
It is RECOMMENDED that operators not generate any new announcements
containing AS_SETs or AS_CONFED_SETs. If they have already announced
routes with AS_SETs or AS_CONFED_SETs in them, then they SHOULD
withdraw those routes and re-announce routes for the component
prefixes (i.e., the additional specifics of the previously aggregated
prefix) without AS_SETs in the updates. This involves undoing the
aggregation that was previously performed (with AS_SETs), and
announcing more specifics (without AS_SETs). Route aggregation that
was previously performed by proxy aggregation (i.e., without the use
of AS_SETs) is still possible under some conditions. As with any
change, the operator should understand the full implications of the
change.
It is worth noting that new technologies (such as those that take
advantage of the "X.509 Extensions for IP Addresses and AS
Identifiers" [RFC3779]) might not support routes with AS_SETs/
AS_CONFED_SETs in them, and may treat as infeasible routes containing
them. Future BGP implementations may also do the same. It is
expected that, even before the deployment of these new or future
technologies, operators may filter routes with AS_SETs/AS_CONFED_SETs
in them. Other than making that observation, this document is not
intended to make any recommendation for how an operator should behave
when receiving a route with AS_SET or AS_CONFED_SET in it. This
document's focus is entirely on the sender side, as discussed in the
preceding paragraph.
Kumari & Sriram Best Current Practice [Page 3]

RFC 6472 AS_SET, AS_CONFED_SET Use Deprecation December 20114. Security Considerations
This document discourages the use of aggregation techniques that
create AS_SETs. Future work may update the protocol to remove
support for the AS_SET path segment type of the AS_PATH attribute.
This future work will remove complexity and code that are not
exercised very often, thereby decreasing the attack surface. This
future work will also simplify the design and implementation of the
Resource Public Key Infrastructure (RPKI) and systems that will
rely on it.
5. Acknowledgements
The authors would like to thank Tony Li, Randy Bush, John Scudder,
Curtis Villamizar, Danny McPherson, Chris Morrow, Tom Petch, and Ilya
Varlashkin, as well as Douglas Montgomery, Enke Chen, Florian Weimer,
Jakob Heitz, John Leslie, Keyur Patel, Paul Jakma, Rob Austein, Russ
Housley, Sandra Murphy, Steve Bellovin, Steve Kent, Steve Padgett,
Alfred Hoenes, Alvaro Retana, everyone in the IDR working group, and
everyone else who provided input.
Apologies to those who we may have missed; it was not intentional.
6. References6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References
[Analysis] Sriram, K. and D. Montgomery, "Measurement Data on AS_SET
and AGGREGATOR: Implications for {Prefix, Origin}
Validation Algorithms", SIDR WG presentation, IETF 78,
July 2010, <www.antd.nist.gov/~ksriram/
AS_SET_Aggregator_Stats.pdf>.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System
(AS)", BCP 6, RFC 1930, March 1996.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779, June 2004.
Kumari & Sriram Best Current Practice [Page 4]